Acinetobacter baumannii is a Gram-negative, non-lactose-fermenting organism that causes serious nosocomial infections such as ventilator-associated pneumonia and bacteremia. A. baumannii is characterized by its tendency to acquire resistance to various classes of antimicrobials that are otherwise effective against this organism. In Pittsburgh, there is a surge in imipenem-resistant and amikacin-resistant A. baumannii, necessitating use of salvage agents such as colistin and tigecycline. Despite the potential magnitude of the problem caused by these multidrug-resistant (MDR) A. baumannii strains, little is known about the mechanisms of resistance to these antimicrobials in A. baumannii in the United States. Current epidemiological studies using pulsed-field gel electrophoresis (PFGE) reveal that A. baumannii causes nosocomial outbreaks, spreading from patient to patient mostly by contact with the contaminated hands of healthcare workers or the hospital environment. A molecular typing method that is more standardized and objective than PFGE is needed for use in surveillance studies to investigate national and international epidemiology of A. baumannii. The hypotheses of the application are: (1) OXA-type carbapenemases (e.g., OXA-23) and ArmA 16S ribosomal RNA methylase are the primary mechanisms by which A. baumannii develops resistance to imipenem and amikacin, (2) The multi-locus variable number of tandem repeats analysis (MLVA) scheme of molecular typing accurately and reliably discriminates clinical strains of A. baumannii and offers advantages over PFGE. To address these hypotheses, the following specific aims have been formulated: (1) Characterize the molecular mechanisms of imipenem and amikacin resistance in MDR A. baumannii clinical strains, (2) Evaluate objective molecular typing methods of A. baumannii. Typing results for MLVA will be compared with PFGE and MLST. The proposed study will enhance the level of understanding in how A. baumannii acquires multidrug resistance and lead to a strategy to best utilize the existing classes of antimicrobials to manage the infections. This will also form the basis for efforts in designing novel therapeutic agents and strategies that will overcome or evade the existing and emerging resistance mechanisms. Objective molecular typing methods will serve as an essential tool for future studies that will address the global epidemiology of MDR A. baumannii. RELEVANCE (See instructions): Acinetobacter baumannii is a problematic pathogen that causes serious infections in patients admitted to hospitals. The project aims to examine how it acquires resistance to two important antibiotics, imipenem and amikacin, and also develop an objective method to "fingerprint" the strains. Such knowledge will help in optimizing use of existing antibiotics and improving infection control practices.